Though built primarily for music production, I also use this rig for gaming quite often as well (and yeah, an i7 5820k just for gaming IS overkill). Never had any issues with latest games such as GTA V and The Witcher 3 in 1080p (using a 22" LCD), don't need/intend to go beyond that any time soon anyway. In some extreme cases, I did need to tweak a thing or too, but that's mainly due to the poor optimization of the game (Crysis 3, AC:Syndicate).
As for audio processing, it works great with Kontakt Sample libraries, some of which are stored an the SSD, while the reast is spread across the two HDDs. With the huge computing power of the 5820k, stacking up many effects like EQs and reverbs is not a problem.
Side notes: because of the lower number of supported PCI lanes, the 5820k is pretty much only suitable for single GPU builds. Not a problem for me, if my GPU starts having trouble with games, it is usually a signal for an upgrade, and buying another piece of the current GPU instead of selling it and buying a completely new one has always seemed the worse option to me anyway. Never been a fan of SLI/Crossfire for gaming anyway as compatibility issues still do occur in many games, and it's not very easy on energy consumption, either.
Comments on the platform: the 5820k is a very powerful and alluring CPU, and, which is not usual with i7s, offers a quite good price/performance ratio, comparign favorably for example with the popular 6700k. The problem is that if you are on some sort of a budget, which most of us are, this CPU is going to bleed your wallet dry because even the "entry level" motherboards like mine (AsRock eXtreme4) still cost around 250$. Along with the price of the CPU, that is already a lot, an in the context of most/gaming builds, way too much. Don't get me wrong, it's not a problem to burn even more on a mobo, but frankly, if you're going to pick even an i7-6700k and overclock it reasonably with a great aftermarket cooler, you'll still get MORE than you actually need for the latest games, you don't need a super expensive motherboard that screams "GAMING PRO" all over it. Usually, it's just a marketing scam, and a lot of mobos from reliable manufacturers can be bought for half the price of a typical X99 board , or even less. In my previous build, I ran an overclocked i5-4570k on a 100$ board and never had any problems with stability.